1. Field of the Invention
The field of the invention is AC motor drives for variable speed control of AC induction motors, and more particularly, AC motor drives using pulse width modulation (PWM) techniques.
2. Description of the Background Art
A motor drive for an AC induction motor includes a power section and a logic and control section. The power section receives power from a 3-phase AC source operating at 60 Hz frequency. The AC power is converted to DC power to provide a PWM inverter with a source for synthesizing voltages of different frequencies which are necessary to control the speed of an AC motor.
The effective value of the output pulses from a PWM inverter approximates one cycle of a sinusoidal AC waveform. The pattern is repeated to generate additional cycles of the AC waveform.
Signals from the logic and control section of the motor drive are applied to the PWM inverter to control the frequency and magnitude of AC power signals to the motor.
In one type of open-loop PWM motor control, speed commands are translated into torque commands by applying a specified volts/hertz ratio, which can be selected through a user-selectable switch or a jumper wire interfaced to the logic and control section of the motor control. Frequency is determined by speed profiles, referred to as "accel/decel" rates which are selected and adjusted through switches interfaced to the logic and control section of the motor control. Open loop controls of this type are moderate in cost and are widely used in industry for applications requiring low and medium horsepower, and moderate speed range.
In a typical AC open loop motor control, a PWM mode of operation is provided during starting and when running in the constant torque range of operation below base speed. At the upper end of its speed range, in the constant horsepower range of operation, the control operates with a six-step square wave output. PWM operation helps reduce harmonics and torque pulsations at the lower end of the speed range.
The PWM region of operation may also be considered a linear region of operation for the gain of the PWM inverter which determines how much output voltage is applied to the motor in response to a motor voltage command input to the inverter. In the linear region, sine-triangle modulation is complete, and a modulation index (Mi) is less than unity. There is a transition region or pulse dropping region, in which the sine-triangle modulation required for PWM operation is less effective and some pulses of the carrier wave are not effective to modulate the motor voltage command. This transition region precedes a six-step square wave mode of operation for the inverter.
In the non-linear region of inverter operation, the inverter gain drops significantly. While investigations have been made into solving the problem of operation in this region, the present invention provides a simplified model for on-line motor control as transition is made from PWM to sixstep operation.